The invention refers to the field of ophthalmology, specifically refractive eye diagnostic and eye surgery. For most refractive eye treatments
(1) pre-surgery diagnostic information of the patient's eye is determined to choose the adequate procedure (e.g. implant vs. laser) and define the individual treatment steps (e.g. where to cut or how to align the implant),
(2) the individual surgery treatment is performed inserting refraction correcting implants (e.g. IOL's, corneal inlays) or executing surgery actions (e.g. cut incisions, apply laser shot patterns) and
(3) post-surgery diagnostic information of the patient's eye including implant and/or surgery action is determined.
(1) and (3) are typically performed outside the operation room using diagnostic devices like keratometer, topographer, wavefront analyzer, scheimflug devices, interferometer or slit lamps. (2) is typically performed in the operation room using a general purpose surgical microscope and adequate tools to support the surgeons manual work (e.g. knifes, phaco machine) or using dedicated devices for partial or full automation of surgical steps (e.g. refractive excimer laser treatment, cataract laser treatment).
Currently there is a wide range of diagnostic devices that measure properties of the eye. A topograph or keratometer determines the shape and curvature of the patient's cornea (e.g. Zeiss Atlas), a wavefront device determines the full refraction of the patient's eye optics (e.g. AMO Wavefront Sciences COAS), an interferometer measures the axial length of the patient's eye ball (e.g. Haag-Streit LenStar LS900), a scheimflug device measures the front-side and back-side of the corneal refraction as well as the thickness (e.g. Oculus Pentacam) and a slit lamp provides an image of the patient's front of the eye for manual examination by the doctor.
All different diagnostic approaches and associated devices evolved to accurate tools with a high repeatability for single eye measurements and therefore are applied pre-surgery as well as post-surgery for examination to verify clinical outcome.
There are further approaches appearing on the ophthalmology landscape for intra-surgery measurement of the eye. An intra-surgery keratometry hand tool (e.g. astigmatic ruler by STORZ) can be used to roughly measure the corneal shape and its changes during the surgery, an intra-surgery wavefront device—in principle—allows the determination of the required power and astigmatism of an artificial lens after the removal of the natural lens (e.g. Wavetec ORange). All intra-surgery refraction measurement tools suffer from the moment of taking the measurement: The moment of eye surgery. Intra-surgery the eye properties are changed compared to the natural no-surgery condition. The intra ocular pressure might be higher, the cornea might be deformed due to mechanical impacts, the refraction of eye fluids changed due to partial exchange of fluids, etc. But independent from this general drawback, the repeatability of those devices in one moment on one specific eye is reasonable.
All named devices and tools in this section above have in common the availability of a more or less consistent intra-device coordinate system (“device-consistent” which means that the tool or device provides from a patient X measured at one moment T multiple times a consistent output) but they all lack a full process covering consistent coordinate system (“process-consistent”). With a process-consistent coordinate system every process step (measurement or treatment) where the patient's eye is visually acquired, can be matched and transformed to an initially defined reference coordinate system.
Due to the lack of a process-consistent coordinate system, systematic errors that occur between different steps are directly impacting the overall treatment error. Some examples:
a) Sit-to-Sit-Error: Current practice is making all diagnostic measurements with the patients head is in an upright position. The assumption of 99% of surgeons is that the gravitation keeps the eye in the exact orientation for every measurement. This way a combination of measurement results from different devices can easily be performed. Unfortunately this assumption is wrong. The eye can rotate up to 7° from one sitting position to another.
b) Marker-Error: Current practice is the use of ink markers or ink marker tools for marking axes or positions on the cornea or the limbus border. The accuracy for using ink markers is limited due to the size of the marker (e.g. can be a 5° thick mark), the unknown coordinate system while the surgeon is doing the marking (see a)) as well as the accuracy of reading a marker. The errors can easily sum up to 6° or more.
c) Surgeons-Error: Till now e.g. the cataract surgeon is doing most surgery steps that require special accuracy fully manual: They position incisions or align implants based on the marks they did previously. Besides the Maker Error the mechanical precision of the surgeon fingers needs to be taken into account.
d) Implant-Error: Depending on the type of implant different post-surgery movements of the implant are likely to occur. For example early toric IOL designs tend to move post-operatively up-to 10° based on slit lamp assessment.
Deriving guidelines, nomograms or new implant designs and tool designs from the overall clinical outcome a separation of different systematic error influences like a)-d) could not be determined or distinguished.
With the high optical complexity of latest generation implants or latest generation laser systems this demand for more diagnostic and surgery accuracy is already present, but with existing tools only overall errors can be determined but no error propagation addressing every single diagnostic step or surgery step.